MULTIPROTOCOL LABEL SWITCHING Muhammad Abdullah Shafiq

MPLS Multi-Protocol Label Switching Label Based Packet Forwarding Forwarding Equivalence Class (FEC) Each FEC maps a Label

W HY MPLS ? T RADITIONAL IP F ORWARDING VS. MPLS BASED FORWARDING Traditional IP based Forwarding Based on Longest Prefix Match Inefficient, slow searching

MPLS B ASED F ORWARDING R1 – R4 : Label Switched Path Labels : Indexes Searching is based on labels which are indexes to forwarding table.

O THER A DVANTAGES OF MPLS Forwarding requires Label lookups and replacements Simple switches can do it. Explicitly routed LSP Vs. Source Routing Label ~ Route No need to carry identity of route with packet.

L ABEL E NCAPSULATION

MPLS D OMAIN Ingress Router Handles traffic entering MPLS Domain Egress Router Handles traffic leaving MPLS Domain Label Switched Path (LSP) Path traversed by packet Label Switching Router (LSR) Forwards packets based on labels.

L ABEL B INDING FEC ~ Label binding (Forwarding Equivalence Class) Upstream & Downstream LSR (w.r.t. binding) If Packet travels : Ru Rd FEC F ~ Label L Ru: Upstream LSR Rd: Downstream LSR

L ABEL A SSIGNMENT & D ISTRIBUTION Downstream Assigned Label binding is assigned by downstream LSR Upstream Distribution Label bindings are distributed upstream Label Distribution Protocols (LDP) To inform the ‘peers’ about label bindings To learn each other’s MPLS capabilities May use TCP for reliability

L ABEL A SSIGNMENT & D ISTR. (C ONTD.)

L ABEL S TACK A labeled packet may carry more than one label. Arranged as Last In First Out (LIFO) stack Processing is based on top label. Unlabelled packet ~ empty packet stack Top label: level m Bottom Label: at level 1 Uses: LSP tunnel and MPLS hierarchy

L ABEL STACK OPERATIONS PUSH Insert a label on top of stack E.g when forwarding a packet that arrives unlabeled but is to be forwarded with label. POP Remove the top label e.g. at egress router SWAP Replace the top label with new label. Normal forwarding between LSRs.

T HE F ORWARDING TABLE

Invalid incoming labels An LSR with no binding of incoming label. Option 1: forward as unlabeled packet Upstream and downstream forward to each other. May cause a loop. Option 2: Discard packet.

LSP C ONTROL What about FEC that corresponds address prefixes learned via dynamic routing ? Two solutions: Independent LSP Control Each LSR binds independently Distributes that bindings to peers Ordered LSP Control Only egress LSR for FEC binds and distributes Used when some properties in paths followed are desired (e.g. explicit specified path)

LSP R OUTE S ELECTION Hop by Hop Each node chooses next hop independently Explicitly routed A single LSR specifies the LSRs in LSP

A GGREGATION Traffic in multiple FECs may go to same egress. Combining multiple FECs to single FEC. Use single label for union of FECs. Aggregation reduces the number of labels.

T UNNELING Ru acts to deliver packets to Rd when they are not consecutive routers. Tunnel from Ru to Rd Tunneled Packet Two methods: Hop by hop routed tunnel Explicitly routed tunnel

H IERARCHY LSP R1, R2, R3, R4 Suppose R2 and R3 are not connected directly R2, R21, R22, R23 R3 forms internal tunnel. Actual sequence: R1, R2, R21, R22, R23, R3, R4

QUESTIONS